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One of five types of synovial joints in the human body, a saddle joint is the connection of two articulating bones, the adjacent surfaces of which are mirror images of each other but positioned perpendicularly to one another. Also known as a sellar joint, it is so named because the adjoining surfaces at the ends of each bone are saddle-shaped, with the saddles meeting to form an X. As each possesses both concave and convex surfaces that curve around each other, it is said that the joint achieves articulation by reciprocal reception. This simply means that the joint moves as the convex surface of the inserting bone moves around the concave surface of the receiving bone and vice versa.
The only synovial joint in the body with the distinction of being considered a saddle joint is the carpometacarpal (CMC) joint in the thumb. It is located roughly 1 inch (2.54 cm) above the wrist at the base of the thumb, between the carpal bones and the metacarpal bones. Due to the distinct shape of this joint, it is said to permit movement in two planes — the sagittal, or front-to-back plane, and the frontal, or side-to-side plane. When the hand is in anatomical position — that is, when the arm is at the side with the palm facing forward — any movement of the thumb forward and backward would be occurring in the sagittal plane, and any movement outward and inward would be occurring in the frontal plane.
The movements allowed by the CMC joint include flexion and extension; adduction and abduction; circumduction, a circling motion, and opposition, or the touching of the thumb to the tips of the other four fingers. At most joints, flexion and extension occur in the sagittal plane, or in a front-to-back motion, but at the thumb flexion and extension occur in the frontal plane as an inward and outward motion, respectively. Likewise, abduction and adduction of this joint occur not in a sideways motion but in the sagittal plane as a front-to-back motion, with the thumb moving out in front of and back in toward the palm.
Circumduction of the CMC joint occurs in both planes. In other words, as the thumb travels through space in a circling motion, it crosses the frontal plane twice and the sagittal plane twice as it completes 360 degrees of movement. Opposition also requires a partial circling motion or arc as the thumb moves inward to make contact with each of the four fingers, and therefore it also occurs in both planes of movement.